1. Field of the Invention
This invention relates to optical lenses, and more particularly relates to wide angle optical ball lenses.
2. Description of the Prior Art
Several major design criteria govern a lens design. These include field of view requirements for a particular application, i.e., the area visible through the lens, lens light gathering ability and lens image quality over a predetermined spectral range. Considerations critical to a lens designed for a wide field of view include variable or fixed focal length and variable or fixed f-number. The lens focal length (f') is the distance between the point of focus of light from an object side of the lens and the plane of the lens. F-number (the relative aperture) is the ratio of the diameter of the focal length of the lens to the entrance pupil (.phi..sub.EP): EQU f--number=f'/.phi..sub.EP):
The entrance pupil diameter is the aperture diameter of the lens diaphragm viewed from the object side of the lens.
A lens design for a specific application requires a lens designer to consider the relationship between lens focal length, lens light gathering ability and acceptable levels of optical aberration for the given application. For example, there is a tradeoff between entrance pupil (light collection diameter) and f-number for a fixed lens focal length. Also, lens images typically vary in intensity, decreasing from the center to the edge of the image, an optical aberration called vignetting. Vignetting occurs when angles of incident light from the object side enter the lens at field angles other than 0 degrees, i.e., off-axis incidence. The result of such off-axis incidence in a lens subject to vignetting is an image projected on a flat focal plane that fades off gradually, i.e., loses intensity, towards the peripheral image edges.
Astigmatic aberration is a lens defect (optical aberration) in consequence of which rays from a point source fail to meet at a focal point. Astigmatic aberration, like vignetting, results in a blurred or imperfect image. Varying the focal length and f-number of many wide field of view lens results in varying degrees of astigmatic aberration. Coma is an optical aberration affecting lens image quality where the image of a point source manifests itself as a comet shaped blur.
Traditional lenses for both photographic and video applications are typically designed to operate in the visible light spectral range. Wide field angle lenses used in the photographic industry are typically designed for use in the visible spectral range of 450 to 650 nanometers (nm). Charge coupled device (CCD) based imaging systems operate beyond the visible light range, displaying sensitivity within the near infrared spectral range. Accordingly, their performance in a spectral range extending beyond 650 nm, i.e., the near infrared (IR) range (600 to 1000 nm) is inadequate due to chromatic aberration.
Chromatic aberration is an optical aberration which greatly affects lens image quality over an extended spectral range, resulting from the fact that different transparent substances refract different wavelength light differently. Longitudinal chromatic aberration occurs when a lens fails to focus light of varied wavelengths within the same plane. Lateral chromatic aberration is a result of lateral image shifts in differing amounts for various light wavelengths, i.e. like a rainbow.
In order to utilize the extended spectral range sensitivity of a CCD array while minimizing chromatic aberration, conventional lenses used therewith are usually complex, typically comprising multiple elements. Even more complex are wide field of view lenses for use with CCD array sensors. Wide field of view camera lenses generally require at least five elements and are consequently large and expensive, the cost of the lens generally increasing with the number of elements.
One example of a commercially available, high quality, wide field angle lens is the NIKON.TM. AF Nikkor 50 mm f/1.4 lens, product no. 1903 manufactured by Nikon Inc. of Melville, N.Y. The NIKON.TM. 1903 lens is based on a complex design, containing 7 elements. Upon testing, the lens was found to exhibit unacceptable levels of chromatic aberration in the near infrared spectral range.
Schneider Corporation of America of Woodbury, N.Y., manufactures a commercial wide field of view lens
operable within the near infrared range -- the XENOPLAN.TM. 17 mm, f/1.4. The Schneider XENOPLAN.TM. lens is, like the above-mentioned Nikkor 1903, extremely complex. The XENOPLAN.TM. lens contains 5 elements and exhibits unacceptable chromatic aberration at the image edges for light entering the lens at wide field angles. The XENOPLAN.TM. is therefore inadequate for certain applications.
Most of the complexity found in modern lens designs is a result of efforts to reduce chromatic aberration with increasing field angle while maintaining consistent light gathering capability. In addition to such efforts, elements may be added to the design of most commercially available wide field of view lenses to enhance their versatility. The cost of such added features and complexity however leads to a lens having far too many features for use in just one specific lens application.